Known legacy inertial reference unit products may employ a Hemispherical Resonator Gyro (HRG), and use digital control loop algorithms, which operate in two, distinct modes: Force to Rebalance (FTR) and Whole Angle (WA). The former mode provides the highest performance angular rate data by caging the HRG resonant standing wave by rebalancing it with electrical force. FTR mode is only effective over a limited angular rate range. The latter mode allows the standing wave to precess with inertial motion, but yields lesser performance. It is effective over a large angular rate range, and is the reversionary mode of operation once the FTR authority range has been exceeded.
Amplitude, phase lock and quadrature control are essential for generating and maintaining the standing wave in either WA or FTR modes. The operation/interaction is as follows: the phase lock loop is a feedback loop, which detects and converges on the unique resonant frequency and phase of the gyro. The output of this control loop generates a specific frequency control to the digital signal processing which maintains the correct rate and phase of the nodal and antinodal sampling. It also is essential for the active controls (rate, and amplitude) to apply their respective outputs with the correct phase offset. In FTR mode, the rate drive is applied with an optimized phase correction, which remains essentially fixed. This optimized value ensures minimal rate energy is introduced into the quadrature. In the case of the forced rotation of the standing wave, as occurs when the rate drive slews the pattern to recover rebalance operation (WA to FTR transition), this rate phase relationship is no longer fixed and large quadrature errors are introduced. These errors are beyond the authority of the standard quadrature control function to null.
Thus, there is a need for an improved hemispherical resonator gyro and, in particular, for improved compensation regarding the above-identified error.